The present invention relates to a rice polishing machine of a vertical shaft and frictional type for polishing rice grain fed from one end of a vertical polishing chamber and discharging it from another end thereof.
A conventional rice polishing machine of a vertical shaft and frictional type will be described with reference to FIG. 9 and FIG. 10. Denoted at reference numeral 37 is a rice polishing machine of a vertical shaft and frictional type comprising a substantially vertical polishing tubular member 38 having a perforated wall and a main shaft 39 rotatably disposed in the polishing tubular member, the main shaft 39 having a bottom section fitted with a spiral rotor 40 and an upper section fitted with a polishing rotor 42 having agitating projection ridges 41 formed on it. Rice grain before polishing is fed to the spiral rotor 40 from a rice grain feed section 45 disposed on a side wall of a machine frame through a rice grain feed duct 43 and fed upwardly to a polishing chamber 46 by the spiral rotor 40. The rice grain is subject to the polishing action caused by the rotation of the polishing rotor 42 to be polished in the polishing chamber 46 and the polished rice grain is discharged from a discharge port 47 through a discharge duct 48 to the outside of the machine.
As shown in FIG. 10, however, with the conventional rice polishing machine of a vertical shaft and frictional type, the polishing rotor 42 is formed such that the distance r.sub.1 between the center axis of the polishing rotor 42 and a part of the peripheral surface of the polishing rotor, which is forward of each of the agitating projection ridges 41 as viewed in the rotating direction (the direction of the arrow) of the polishing rotor 42, is larger than the distance r.sub.2 between the central axis of the polishing rotor 42 and a part of the outer peripheral surface, which is rearward of each of the agitating projection ridges 41. As a result, the polishing pressure applied to a section A in the polishing chamber becomes excessive larger than the polishing pressure applied to a section B. Even though the polishing pressure applied to the section A is properly adjusted, the pressure in the section B is made unnecessarily smaller so that the rice grain in the section B rapidly ascends along the agitating projection ridges in an insufficiently polished condition and discharged, resulting in that the rice grain is polished irregularly On the other hand, when the polishing pressure applied to the section B is properly adjusted, an excessive pressure is applied to the section A, which produces broken grain.